A hybrid vehicle refers to a motor vehicle that uses an electric motor and an internal combustion engine in combination. In the case where the drive load of the hybrid vehicle is large, for example, when the hybrid vehicle is driven at a high speed or when the hybrid vehicle is driven on an uphill road, the hybrid vehicle is operated in an engine drive mode in which the internal combustion engine is used.
Conversely, in the case where the drive load of the hybrid vehicle is small, for example, when the hybrid vehicle is driven at a low speed or when the hybrid vehicle is stopped, the hybrid vehicle is operated in a motor drive mode in which the electric motor is used.
Such a hybrid vehicle is provided with an air conditioning system for cooling or heating a vehicle room. As illustrated in FIG. 1, the air conditioning system includes an air conditioner case 1 in which a blower 3, an evaporator 5, a heater core 7 and a temperature door 8 are installed.
The blower 3 is configured to draw an internal air or an external air and to blow the same toward an internal passage 1a of the air conditioner case 1. The evaporator 5 is configured to cool the air blown toward the internal passage 1a. The heater core 7 is configured to heat the air blown toward the internal passage 1a. The temperature door 8 is rotated between a cold air passage 1b and a hot air passage 1c to adjust the opening degree of the cold air passage 1b or the hot air passage 1c. 
In particular, the heater core 7 is configured to receive hot cooling water from an engine 9, thereby allowing the hot cooling water to exchange heat with the ambient air. Thus, the heater core 7 heats the air blown into the vehicle room, consequently heating the vehicle room.
In this regard, an electric water pump 9a is installed between the heater core 7 and the engine 9. If the motor vehicle is switched from an engine drive mode to a motor drive mode, the electric water pump 9a forcibly pumps the cooling water of the engine 9 and forcibly circulates the cooling water through the heater core 7.
Thus, when the engine 9 is stopped as the motor vehicle is switched from the engine drive mode to the motor drive mode, the electric water pump 9a prevents the circulation of the engine cooling water through the heater core 7 from being stopped due to the stoppage of the engine 9, consequently preventing reduction of heat generation in the heater core 7.
Accordingly, during the motor drive mode, the engine cooling water is continuously circulated through the heater core 7 even if the engine 9 is stopped. As a result, even when the engine 9 is stopped, the heater core 7 continues to generate heat, thereby continuously heating the vehicle room.
In general, the electric water pump 9a is configured so that the rotational speed thereof is automatically controlled depending on the rotation speed level of the blower 3.
The reason for employing this configuration is to control the rotational speed of the electric water pump 9a depending on the thermal load of the heater core 7 which corresponds to the rotation speed level of the blower 3.
In this way, the heat generation amount of the heater core 7 is controlled depending on the thermal load of the heater core 7, whereby the temperature of the air discharged into the vehicle room is automatically controlled depending on the thermal load of the heater core 7.
However, the air conditioning system of the prior art has a structure in which the rotation speed level of the blower 3 is not accurately controlled depending on the thermal load of the heater core 7. Thus, the electric water pump 9a controlled based on the rotation speed level of the blower 3 is not accurately controlled depending on the thermal load of the heater core 7. This poses a problem in that the rotational speed of the electric water pump 9a may be set too high or too low as compared with the thermal load of the heater core 7.
Specifically, the rotation speed level of the blower 3 is controlled at about eight levels depending on the thermal load of the heater core 7. This makes it impossible to accurately control the rotation speed level of the blower 3 depending on the thermal load of the heater core 7. Thus, the electric water pump 9a controlled based on the rotation speed level of the blower 3 is not accurately controlled depending on the thermal load of the heater core 7.
For that reason, there may be a case where the rotational speed of the electric water pump 9a is too high or too low as compared with the actual thermal load of the heater core 7. This may pose a problem in that the amount of the engine cooling water circulated through the heater core 7 is excessively increased or decreased.
Particularly, if the rotational speed of the electric water pump 9a is too high as compared with the thermal load and if the amount of the engine cooling water circulated through the heater core 7 is excessively increased, the heat generation amount of the heater core 7 grows higher as compared with the thermal load. Thus, the temperature of the air discharged into the vehicle room is increased, thereby making the vehicle room unpleasant.
In addition, if the amount of the engine cooling water circulated through the heater core 7 is excessively increased, the temperature of the engine 9 is prematurely decreased. For that reason, when restarting the engine 9, the operation efficiency of the engine 9 may be decreased. As a result, the fuel efficiency of the motor vehicle may be reduced.
On the other hand, if the rotational speed of the electric water pump 9a is too low as compared with the thermal load and if the amount of the engine cooling water circulated through the heater core 7 is excessively decreased, the heat generation amount of the heater core 7 grows lower as compared with the thermal load. Thus, the temperature of the air discharged into the vehicle room is decreased, thereby deteriorating the pleasantness of the vehicle room.